Introduction: Small red lights (≤ 1 min arc) can appear white or yellowish with small amounts of positive defocus under daylight conditions (Wood et al., 2005). We investigated the influence of longitudinal chromatic aberration and monochromatic aberrations on the appearance of small red lights.

Method: Seven cyclopleged, colour normal observers viewed a red light (1′ arc, 6,250 cd.m-2, dominant wavelength 628 nm) centred within a black annulus (4.5′ arc) and surrounded by a uniform white field (2,170 cd.m-2). Effective pupil size was 4 mm. An optical trombone varied focus. Longitudinal chromatic aberration was controlled with a Powell achromatizing lens that neutralizes the eye's chromatic aberration. It has two components, a doublet that doubles the eye's chromatic aberration, and a triplet that reverses the eye's chromatic aberration. The eye's astigmatism and higher order monochromatic aberrations were corrected using adaptive optics.

Results: Observers reported a change in appearance of the small red light without the Powell lens at +0.49±0.21D defocus (mean ± SD) and with the doublet component at +0.62±0.16 D. The appearance did not alter with the Powell lens, and 5/7 observers reported the phenomenon with the triplet component for negative defocus (-0.80±0.47 D). Monochromatic aberration correction did not significantly affect the magnitude at which the appearance of the red light changed (+0.44±0.18 D without correction; +0.46±0.16 D with correction).

Conclusion: Longitudinal chromatic aberrations, but not monochromatic aberrations are involved in the change of appearance of small lights with defocus.